Aeration of waste water is known and involves introducing air into the water and mixing the air and water to promote biological consumption of algae and other pollutants present in the water.
Various mechanical aeration devices have been proposed utilizing a submerged propeller coupled to a draft tube. Rotation of the propeller agitates the water and creates a differential pressure which draws air through the draft tube for discharge into the water. To treat water effectively and efficiently with such mechanical devices, it is desirable to introduce into the water as much air as possible per unit of time and as a dispersion of very small bubbles. It also is desirable to produce as much thrust as possible from the propeller to force the air bubbles deeply into the water for optimizing retention time of the air bubbles in the water. Preferably this is accomplished with minimum consumption of energy.
Known mechanical aerator devices have included hollow hub-type aerators such as those disclosed in U.S. Pat. Nos. 4,280,911; 4,308,221; 4,954,295; and 4,741,870. Each of these devices includes rotatable propeller blades for generating low pressure in the vicinity of the propeller hub causing air to be drawn through a draft tube and out of the hub for discharge into the water. Such devices require rotation of the propeller at high velocity in order to generate sufficiently low pressure to draw air through the tube. Rotating the propeller at high speeds results in high energy consumption and produces a rather coarse dispersion of air bubbles in the water which negatively affects aeration efficiency.
Other aeration devices have been proposed utilizing propellers having perforated hollow blades communicating with atmosphere through a draft tube. Like the hollow hub aeration devices described above the hollow blade devices conduct the air or other fluid to the liquid under the influence of suction generated by the rotation of the propeller in the liquid. Since the blades rotate at a relatively higher circumferential velocity than the hub, increased aeration can be produced by providing outlet ports in the blade rather than in the hub. Examples of known hollow blade aerating devices appear in U.S. Pat. Nos. 4,200,597; 4,371,480; and 5,013,490.
In the operation of aerator devices of the kinds described above, positive air pressure at the air outlets produced as a result of rotation of the propeller prevents the backing of liquid into the draft tube through the outlets. When the propeller is not rotating, however, water can enter the draft tube via the air outlets. Such a result is particularly objectionable when treating waste water of a sewage treatment facility since, over time, algae and other bacterial matter accumulate in the draft tube and block the air passages.
Waste water also may be so corrosive as to damage bearings and seals within the draft tube. In order to avoid such problems, it presently is necessary manually to dry dock or remove the aerator from the water so that the air outlets are above the surface of the water.
Some manufacturers of aeration devices have replaced conventional antifriction bearings with water bearings as a means for extending the bearing life of the aerator. Wet bearings, however, also are prone to attack by waste water and require replacement after a short period of time. In contrast, ball or roller bearings will last considerably longer than wet bearings if they are adequately protected from exposure to the waste water. The Gross patent referred to above discloses a conventional antifriction bearing journaling the drive shaft, but such bearing is located above the surface of the waste water to prevent its exposure to the water. This construction results in several feet of the drive shaft extending beyond the bearing, thereby requiring support from a cantilever sleeve and additional wet bearings, resulting in additional complexity and cost in the manufacture of such devices.